The Global Crisis has intensified debates over the merits of financial innovation and the optimal size of the financial sector. This column presents a model in which the growth of finance is driven by the development of a financial innovation. The model can help explain the securitised mortgage debacle that triggered the latest crisis, the tech bubble in the late 1990s, and junk bonds in the 1980s. A striking implication of the model is that regulation should be toughest when finance seems most robust and when innovations are waxing strongly.